1. Field of the Invention
The present invention is directed to strain sensors, and more particularly, a solid-state sensor that detects deformation based on the dielectrostrictive response of the corresponding sample under test.
2. Description of Related Art
Strain gauges or sensors have been employed in a wide variety of applications. Conventional strain sensors are typically used for measuring the expansion and/or contraction of an object under stress. A common type of strain sensor comprises a resistive transducer. Other types of strain sensors include air-gap capacitive sensors (including capacitors that simulate adjacent parallel capacitors), piezo resistors (including silicon strain gauges and conductive elastomer resistive strain gauges), piezoelectric devices such as lead zirconium-titanate (PZT), as well as others.
Resistive strain sensors generate the resistance change that is proportional to the amount the object being measured is deformed under strain. In one type of resistive of the object it is mounted on in a predetermined direction. Such a sensor requires either a DC or an AC excitation voltage to generate an electric output signal proportional to the strain. In addition, auxiliary equipment (for example, connecting the sensor in a differential arrangement such as in a wheatstone bridge circuit), typically must be provided to accurately determine the amount of strain.
Capacitance strain gauges, such as those shown in FIGS. 1A and 1B, depend on geometric features of the gauge to measure strain. In FIG. 1A, a capacitance strain sensor 10 includes opposed parallel plates 12, 14 separated by a pair of spacers 16, 18, which collectively define an air gap 20. Or, as shown in FIG. 1B, a capacitance strain sensor may include a single spacer 16 to separate the opposed plates 12, 14, which may be preferred depending on the type of forces being sensed or required mounting configuration. As a compressive load is applied to the sensor, as shown, the separation between the opposed plates changes (e.g., narrows as shown in FIGS. 1A and 1B when subjected to a compressive load), thus causing a change in capacitance. In particular, capacitance, C, of a parallel plate capacitor can be characterized as being proportional to A×K/h where A is the plate area, K is the dielectric constant of the dielectric material; between two plates and “h” is the separation between the plates. As a result, the capacitance can be varied by changing the plate area, A, or the gap, h. The electrical properties of the materials used to form the sensor are generally unimportant, so the capacitance strain gauge materials can be chosen to meet the mechanical requirements of the particular application. Therefore, such sensors are useful in those instances where a more rugged sensor is needed, providing a significant advantage over resistance strain gauges. However, as discussed below, such sensors have drawbacks of their own.
Although useful for certain applications, the above-described sensors have inherent drawbacks. Resistive strain sensors require relatively complex measurement equipment (e.g., a wheatstone bridge), and can have less than ideal robustness. Moreover, resistive strain gauges dissipate a significant amount of heat, thus making their implementation impractical for many applications contemplated by the present invention including, for example, when the sensor is embedded with the object being sensed. Conventional capacitance strain gauges, although more robust than resistance strain gauges, are limited by the range of forces they can sense. For instance, measuring shear forces with a conventional capacitance sensor is difficult. Also, conventional air-gap capacitance strain sensors are not sufficiently sensitive for the applications contemplated by the present invention and, in any event, are vulnerable to overload in the presence of large forces, thus further limiting their application.
Moreover, in a common capacitance strain sensor, capacitance is given byC=∈0∈A/h  Equation 1where ∈0 is the dielectric constant of free space, ∈ is the dielectric constant of the material between the electrodes, A is the electrode area, and h is thickness of the layer. Deformation changes the gap, h, the area, A, and affects the dielectric constant, ∈. The relative variation in the capacitance can be expressed as
                                          Δ            ⁢                                                  ⁢            C                    C                =                              -                                          Δ                ⁢                                                                  ⁢                h                            h                                +                                    Δ              ⁢                                                          ⁢              A                        A                    +                                                    Δ                ⁢                                                                  ⁢                ɛ                            ɛ                        .                                              Equation        ⁢                                  ⁢        2            
The first two terms in Equation 2 represent the contribution of electrode geometry which is well-addressed in the prior art. The last term represents the contribution due to the dielectrostriction effect which is largely overlooked in the sensor art. However, available theoretical and experimental results predict the dielectrostriction effect typically dominates the geometric variations.
Unfortunately, constraints at the interface between the electrodes and the dielectric layer strongly influence the strain-dielectric response, Δ∈/∈, for a parallel-plate configuration. This can be illustrated by considering variations of dielectric properties with deformations. For example, according to Equation 1 uniaxial compression changes both the thickness, Δh/h≈u33, and the volume, ΔV/V≈ull=u11+u22+u33, of the layer. If the dielectric layer is allowed to freely slip between the electrodes, the resulting lateral expansions of the material try to preserve its volume, u11=u22=−vu33, and thus ΔV/V=(1−2v)u33. If the dielectric layer is constrained so no lateral expansions are allowed, it can be compressed only in the z-direction, and thus u33(=Δh/h) and u11=u22=0, which results in Δh/h=ΔV/V=u33. Therefore, the measured variations of the dielectric constant range from Δ∈33=[α1+(1−2v)α2]u33 to Δ∈33=(α1+α2)u33 for various boundary constraints. A vital drawback is that the correct type of the constraints for a given thin-dielectric-layer setup is typically unknown.
In any event, with each of the above-noted sensors, the sensors are external or separate from the object experiencing stress/strain. It follows that direct sensing of stresses or strains through physical response of a material composing the structure itself would be more attractive. Elimination of embedding, attaching, and interfacing external devices reduces cost, power consumption and increases reliability of the system.
One approach to achieving such direct sensing is to embed or laminate a material having an intrinsically self-sensing capability into the load-bearing structure. For example, piezoresistive response of graphite fibers in a polymeric matrix is linked to deformation of the composite. Similarly, a surface-bonded or embedded piezoelectric material or glass-optical fibers can be also utilized for monitoring the deformations. However, introducing self-sensing materials into the load-bearing structure has many shortcomings.
For instance, embedded material increases cost, affects mechanical response and reliability of the structure. Also, stiffness mismatch of the matrix and the embedded elements may result in poor strain transfer and be an additional source of system degradation. In addition, wiring or interfacing embedded sensors increases risks of operational failure of the system, and only few materials possess the piezoelectric, piezoresistive or optical responses required for implementation of the traditional approaches discussed above.
In response to these challenges, the sensor disclosed in U.S. Pat. No. 6,910,385 (“the '385 patent”) to the present assignee was developed. The '385 patent, which is hereby expressly incorporated by reference herein, discloses a solid-state capacitance strain sensor that operates based on variation of dielectric properties with deformation, also known as electrostriction or dielectrostriction. The sensor may be configured as a two-sided device according to the design of a conventional capacitor, or alternatively may be configured as a one-sided device as a line capacitor sensor. Moreover, the dielectric material employed in the sensor may be micro-tailored according to particular applications to increase sensitivity to, for example, shear or normal deformation.
In one preferred embodiment of the '385 patent, the dielectric is the object being sensed while appropriately placed electrodes that interface directly with the object under test measure capacitance changes in response to deformation of the object in response to incident forces. Such capacitance changes are caused by a change in the dielectric constant of the object being sensed as deformation due to forces, for example, shear and/or compressive, are exerted thereon. More particularly, the solid-state capacitance includes at least one pair of electrodes disposed so as to interface with the dielectric. Electrodes can be attached to the dielectric and move following the deformation. The sensor system includes a measuring circuit coupled to the electrodes to measure a change in the dielectric constant in response to the deformation. In operation, the change in the dielectric constant is caused by an electrostrictive response of the dielectric to the strain. The response is quantified by computing a change in the dielectric constant based on a measured change in capacitance.
Though useful, the disclosed embodiments of the '385 patent do not provide an elegant solution to providing a cost-effective sensor for multi-component (three-axis) strain/stress detection. Moreover, with mechanical contact to the monitored part, movement of the electrodes can corrupt the acquired data, while the stand-alone nature of the sensor is susceptible to electrical noise, thermal effects and related signal corruption. Overall, a robust solution was needed for a range of applications including in-line monitoring of various load-bearing structures, stand-alone sensing devices and arrays for stress/strain mapping (for example, tactile sensing), process monitoring of toughened (tempered) glasses, residual stresses in plastic parts, to name a few.